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Large Scale Simulations of Reionization

This presentation delves into large-scale simulations aimed at understanding the reionization epoch of the universe, a critical period when the first galaxies formed. Collaborating researchers from the Stockholm Observatory and LOFAR EoR Key Project team have explored simulation methodologies, including PMFAST and C2-Ray, to illuminate the complex reionization process. Key findings reveal the dynamics of ionized hydrogen fractions and the significance of spatial volumes in reionization histories. Results suggest the need for observational alignment with future initiatives such as LOFAR to refine our understanding of these cosmic transitions.

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Large Scale Simulations of Reionization

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  1. Large Scale Simulations of Reionization Garrelt Mellema Stockholm Observatory Collaborators: Ilian Iliev, Paul Shapiro, Marcelo Alvarez, Ue-Li Pen, Hugh Merz, LOFAR EoR Key Project team

  2. Contents • Reionization • Simulations • Some results • WMAP1 versus WMAP3 • Secondary CMB Anisotropies • Iliev et al. 2006, astro-ph/0512187 • GM et al. 2006, astro-ph/0603518

  3. Reionization • A view of the epoch when the first galaxies formed. • Current observational data: WMAP Thomson optical depth, SDSS QSOs, TIGM from Lyα forest • Future observational data: redshifted 21cm radiation (21CMA, LOFAR, MWA, SKA); direct view of HII regions (nature of sources) and IGM density field.

  4. Simulations: Pro and Cons • Limited ranges • mass resolution • spatial resolution • number of sources • Expensive • Possible saves: • No temperature • No helium • Simple source prescription • Reionization process is complex: • Source clustering • HII region overlap • Recombinations (n2) • Temperature effects • Analytical models cannot capture all of these effects, numerical models can.

  5. Simulations: How? • Our motivation: large scalesimulations. • Observationally needed (~degree fields of view). • Theoretically needed (cosmic variance, size of HII regions, >>10 Mpc). • Approach: • PMFAST(Merz, Pen, Trac2005) simulations (4.3 billion particles): • Evolving density field • Collapsed halo list • 100/h and 35/h Mpc volumes (minimum halo masses 2.5x109 and 108 M, respectively). • C2-Ray(GM et al. 2006) postprocessing (2033, 4063): • Ionized hydrogen fraction ΛCDM (WMAP)

  6. Simulations: Sources • We have been working with stars as our sources of ionizing radiation. • Assumptions: • M/L=const. • only atomically cooling halos contribute (M>108 M). • halos with M<109 M can be suppressed. • fixed photons/atom escaping (Iliev, Scannapieco & Shapiro 2005): f = fSF x fesc xNphoton. • Choices used: f=2000 and 250. • Other source models to be explored in the future.

  7. Results: evolution • Movie of density field and HII regions • Green: neutral • Red: Ionized • Note: clustering & overlap. • From z=20 to 10 (WMAP3 parameters). Overlap expected at z~7. 35/h Mpc

  8. Importance of Large Scales • From our (100/h Mpc)3 volume we can analyze the reionization history of subvolumes. • Large variations found, need at least volume of (30/h Mpc)3. • Reionization is mostly inside-out. Reionization histories for subvolumes

  9. Statistics Full density HII density HI density • 3D powerspectra: • Poisson noise at largest scales • Clear peak at some (time-dependent) characteristic scale. • Resemble analytical work (Furlanetto et al. 2004a,b) • The signal is strongly non-gaussian: • Numerical results do not resemble analytical inside-out, nor outside-in results (Furlanetto et al. 2004a,b) Furlanetto et al. 2004a

  10. z=11.9 z=10.8 20/h Mpc 10/h Mpc 5/h Mpc Statistics • 3D powerspectra: • Poisson noise at largest scales • Clear peak at some (time-dependent) characteristic scale. • Resemble analytical work (Furlanetto et al. 2004a,b) • The signal is strongly non-gaussian: • Numerical results do not resemble analytical inside-out, nor outside-in results (Furlanetto et al. 2004a,b) Furlanetto et al. 2004b

  11. LOS Reionization Histories • From the simulations we can construct reionization histories along the line of sight. • Will be used to prepare for the analysis of the LOFAR observations (2009)

  12. What Cosmology? • 1st year WMAP versus 3 year WMAP: • τ: 0.17: to 0.09 (if instantanious, zreion: 16 to 11) • ns: 1.0 to 0.95, σ8: 0.9 to 0.74. • Reionization happened later (good!), but structure formation also took longer. • Alvarez et al. (2006): approximate scaling for simulations with similar types of sources: (1+z1)/(1+z3)≈1.4. Confirmed by new simulations.

  13. WMAP1 WMAP3 WMAP1 versus WMAP3 • Identical simulations (100/h Mpc, f=250), differing only in cosmological parameters: FM Band FM Band

  14. Secondary CMB Anisotropies • Patchy reionization is expected to imprint small scale anisotropies on the CMB signal through the kinetic Sunyaev-Zel’dovich effect. • Several analytical estimates exist (Hu & Gruzinov 98,McQuinn et al. 2005, Santos et al. 2006, Zahn et al. 2006), with large variation in strength and scales. Now the first numerical ones. • Temperature variations given by LOS integral:

  15. Sample kSZ map from patchy reionization • Sample kSZ map (100/h Mpc, f=250). • Range of pixel values is DT/T=-10-5 to 10-5 , i.e. DT max/min are in the tens of mK at ~arcmin scales. ~1° ~1°

  16. kSZ Power Spectra • Power spectra peak at l~3000-5000, with a peak value ~1 μK. • Instant reionization has order of magnitude less power for l~2000-8000, but same large-l behaviour. • Uniform reionization has much less power on all scales.

  17. Conclusions • Large scale simulations needed for useful results. • Reionization produces a clear signature in the nHI power spectra. • WMAP3 results do not require different types of sources, but move reionization by a factor ~1.4 in (1+z). • kSZ due to patchy reionization produces a signal of ~μK at l~3000-5000.

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